Grafting of unsaturated monomer molecules onto olefin polymers and copolymers has been disclosed in a number of patents. The graftings have been used to impart changes in the polymer to which the grafted molecules are attached.
With respect to the invention described and claimed in this application, it is believed that the following patents are representative of the most relevant prior art on grafting of which we are aware: U.S. Pat. Nos. 2,970,129; 3,177,269; 3,270,090; 3,873,643; 3,882,194; 3,886,227; 4,087,587; 4,087,588; 4,239,830; 4,298,712; 4,394,485; 2,081,723; Jap. Kokai 49(1973)-129742.
The principal distinctions between low density polyethylene (LDPE), high density polyethylene (HDPE), and linear low density polyethylene copolymer (LLDPE) are well-known to practitioners of the polyethylene art and are described e.g., in U.S. Pat. No. 4,327,009.
There are, basically, two types of olefin polymerization techniques for preparing high molecular weight olefin polymers and copolymers. The oldest commercial technique involves high pressure, high temperature, and the use of a free radical initiator, such as a peroxide in the polymerization of ethylene; these type polymers are generally known as low density polyethylene (LDPE) and are also known as ICI-type polyethylenes. These LDPE polymers contain branched chains of polymerized monomer units pendant from the main polymer "backbone" and generally have densities in the range of about 0.910-0.935 gms/cc.
The other commercially-used technique for making polyethylene involves coordination catalysts of the "Ziegler" type or "Phillips" type and includes variations of the Ziegler type, such as the Natta type. These catalysts may be used at very high pressures, but may also (and generally are) used at very low or intermediate pressures. In this technique, other olefins in the range of C.sub.3 -C.sub.12 or more can be polymerized and copolymerized into high polymers. The products made by these coordination catalysts are generally known as "linear" polymers because of the substantial absence of branched chains of polymerized monomer units pendant from the main polymer "backbone"; the linear homopolymers of ethylene are also generally known as high density polyethylene (HDPE). Linear polyethylene homopolymer (HDPE) ordinarily has a density in the range of about 0.941 to about 0.965 gms/cc.
In some of the embodiments of the present invention there is used a "linear" type ethylene polymer wherein ethylene has been polymerized along with minor amounts of at least one alpha, beta-ethylenically unsaturated alkene having from 3 to 12 carbons per alkene molecule, preferably 4 to 8. The amount of the alkene comonomer(s) is generally sufficient to cause the density of the polymer to be substantially in the same density range as LDPE, due to the alkyl sidechains on the polymer molecule, yet the polymer remains in the "linear" classification; they are conveniently referred to as "linear low density polyethylene" (LLDPE) copolymers. An acronym for linear medium density ethylene copolymers between LLDPE and HDPE is "LMDPE". An acronym for ultra low density ethylene polymer(s) below the so-called "low" range of densities, is called "ULDPE", and by some practitioners it is known as very low density ethylene copolymer, "VLDPE". Thus, the range of densities of linear polymers can range from below 0.87 gm/cc to over 0.965 gm/cc.
The use of coordination-type catalysts for polymerizing ethylene into homopolymers (which we call "HDPE" polymers) and/or for copolymerizing ethylene with higher alkenes to make copolymers (which are called "LLDPE" copolymers) is disclosed variously in, e.g., U.S. Pat. Nos. 2,699,457; 2,862,917; 2,905,645; 2,846,425; 3,058,963; and 4,076,698.